<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
  <style type="text/css">
    body{margin:0;padding:0;}
    .parent{width:400px;height:400px;background-color:#af3;margin:20px 0;}
    .child{width:200px;height:200px;margin:30px 0;background-color:#f3a;}
</style>
</head>
<body>
  <div class="parent">
      <div class="child"></div>
  </div>
</body>
</body>
</html>